I've always been fascinated by car vs. bike track comparisons. Back in the 90's, Sportrider's Kent Kunitsugu took on a Corvette ZR1 with a FZR-1000. Just recently, Fifth Gear's Tiff Needell drove a Ariel Atom 2 against Steve Rapp on the Attack Performance CRT MotoGP bike. Both results were pretty much the same as this comparison.
Boiled down, the bike has a tremendous power-to-weight ratio advantage, which easily overcomes the grip advantage a supercar has with massive contact patch. So in braking the absolute corner speed, the car has the advantage. When any application of throttle is required, it's no contest for the bike. So the fact that most of any lap is spent accelerating, as long as it is dry, the bike is just about always going to win.
Another factor is how much smaller and skinnier a motorcycle is compared to any supercar. Due to this fact, a bike can take tidier lines and ends up covering considerably less distance around the same track than a supercar.
The most impressive thing I've observed in these showdowns is how much advancement there has been in motorcycle tires. The compounds and profiles have developed so much more that I wager this accounts for the greatest amount of time gained over supercars. Cornering speeds in the 90's were something like 10% less than a supercar of the day, now the bikes are nearly matching the apex speeds of supercars while still maintaining their acceleration advantage.